A PRELIMINARY STUDY OF UNIDENTIFIED TARGETS OBSERVED ON AIR TRAFFIC CONTROL RADARS [view cover]
Richard C. Borden, Electronics Division and Tirey K. Vickers, Navigation Aids Evaluation Division Technical Development -Report No, 180
CIVIL AERONAUTICS ADMINISTRATION TECHNICAL DEVELOPMENT AND EVALUATION CENTER INDIANAPOLIS INDIANA May 1953
U. S. DEPARTMENT OF COMMERCE Sinclair Weeks, Secretary
CIVIL AERONAUTICS ADMINISTRATION F. M. Lee, Administrator D. M. Stuart, Director, Technical Development and Evaluation Center
LIBRARY FEDERAL AVIATION AGENCY
Class TL 568 Book A21N Vol. no 180 Copy 3
This is a technical information report and does not necessarily represent CAA policy in all respects.
TABLE OF CONTENTS
Page SUMMARY 1 INTRODUCTION 1 OFFICIAL RECORDS 1 HISTORICAL REFERENCES 4 WASHINGTON OBSERVATIONS 5 ANALYSIS OF WASHINGTON DATA 6 SUPPLEMENTARY OBSERVATIONS AT INDIANAPOLIS 13 ANALYSIS OF SUPPLEMENTARY OBSERVATIONS 14 EFFECT ON AIR TRAFFIC CONTROL OPERATIONS 14 CONCLUSIONS 14 RECOMMENDATIONS 15 BIBLIOGRAPHY 16
The Air Navigation Development Board (ANDB) was established by the Departments of Defense and Commerce in 1948 to carry out a unified development program aimed at meeting the stated operational requirements of the common military/civil air navigation and traffic control system. This project, sponsored and financed by the ANDB is a part of that program. The ANDB is located within the administrative framework of the Civil Aeronautics Administration for housekeeping purposes only. Persons desiring to communicate with ANDB should address the Executive Secretary, Air Navigation Development Board, Civil Aeronautics Administration, W-9, Washington 25, D. C.
A PRELIMINARY STUDY OF UNIDENTIFIED TARGETS OBSERVED ON AIR TRAFFIC CONTROL RADARS
This report describes the investigation of a type of unidentified moving target which has been observed recently in considerable numbers on the viewing screens of air traffic control radar equipment operated by the Civil Aeronautics Administration. This investigation was conducted by means of interviews with personnel concerned, by study and correlation of official records, and by first-hand observation of numerous targets on the Washington Microwave-Early-Warning (MEW) radar and on the Indianapolis ASR-2 radar.
It was determined that targets which are known to operating personnel by various terminologies such as "ghosts," "angels," or "pixies" do not represent new phenomena; nor are they peculiar to the Washington area. Correlation of controllers' reports with United States Weather Bureau records indicated that a surface temperature inversion was almost always noted when such targets appeared on the radar.
Firsthand observation in the tracking and subsequent motion analysis of 80 of these unidentified targets indicated that a large number of these were actually secondary reflections of the radar beam. Apparently these reflections were produced by isolated refracting areas which traveled with the wind at or near the temperature inversion levels.
Although the exact size, shape, and composition of these isolated areas are not known, it is believed that they may be atmospheric eddies produced by a shearing action of dissimilar air strata. It appears possible that such eddies may refract and focus the radar energy with a lens effect to produce small concentrations of ground return with sufficient intensity to show up on the radar display. It is also believed that the correlation of the appearance of these radar targets with visual reports of so-called "flying saucers" is due to the strong probability that both effects are caused primarily by abrupt temperature inversions.
Such radar targets are usually easy to recognize because of their generally weak return and slow ground speed. Unfortunately, radar returns from small helicopters sometimes present these some characteristics. Spurious targets of this type can become a nuisance under busy traffic conditions, particularly in localities where helicopter operations are prevalent.
Closely related to a recent flood of visual reports of flying saucers, the sighting f scores of unidentified targets on the Washington Air Route Traffic Control Center (ARTC) radar aroused much publicity and speculation regarding the origin, composition, and import of these objects. Concerned with the possible detrimental effects of this situation on the control of air traffic, the Air Navigation Development Board requested the Technical Development and Evaluation Center of the CAA to investigate the problem.
The specific objectives of this study were:
1. To find out as much as possible about the nature of the targets themselves. 2. To determine whether this problem is new and peculiar to the Washington area or whether it had occurred previously at Washington and at other CAA radar locations. 3. To determine the effect of this problem on the control of air traffic. 4. To determine what changes should be made in the radar development program in order to cope with the situation.
As one of the first steps in this study, all records of these phenomena reported in the logs of the Washington ARTC Center were tabulated. The tabulation, given as Table I of this report, was taken to the Analysis Section of the United States Weather Bureau where it was correlated with meteorological data for the periods involved. It was then discovered that a temperature inversion had been indicated in almost every instance when the unidentified radar targets or visual objects had been reported. Weather analysts were asked whether any unusual weather conditions had prevailed over the Washington area during the period covering the occurrences of large numbers of the unidentified radar targets. Their report may be condensed as follows:
Monthly Weather Summary, July 1952.
The heat wave that broke records in the eastern portion of the United States during the month of Tune continued on through July, becoming intensified during the latter part of the month. July weather maps were characterized by a well-developed Bermuda high pressure area which remained in the vicinity of the southeastern coast line during the entire period. This high pressure area was responsible for an anticyclonic (clockwise) circulation of air over the eastern United States, a movement which continued during the month. This flow brought warm, moist air up from the Gulf of Mexico. The warm air mass usually extended up to about 10,000 feet. At higher levels the flow was from the west-southwest, and this continental air mass from the southwestern desert and drought area was hot and dry. Stagnation and heating of the air over the eastern United States was further increased because of an extremely strong band of westerly winds along the northern United States border, winds which prevented cold Canadian air masses from pushing south. Cyclonic activity was confined mostly to the area north of this band of westerly winds. There was a notable lack of thunderstorm activity in the Washington area. Physicists at the Naval Observatory reported that the amount of electrification in the air was very low.
The foregoing analysis indicated that the lack of cloud cover promoted solar heating in the daytime and rapid radiation cooling of the surface at night, This combination, with the prevailing light winds, was unusually conducive to the formation of temperature inversions during the hours of darkness.
Since the visual reports of flying saucers indicated that the observed lights spanned the same color range as the aurora borealis and since auroral effects closely follow sunspot activity, personnel of the Naval Observatory were consulted in order to determine whether any unusual sunspot activity had occurred during the period in question. They reported that there had been no unusual activity of this nature.
Reports from Other Locations.
The Washington ARTC Center is the only one equipped with air route surveillance radar. However, several CAA control towers are equipped with airport surveillance radar, Type ASR-1. A survey of these locations produced the following results:
ATLANTA, Municipal Airport. No unidentified targets of this nature have been reported.
BOSTON, Logan Field. Unidentified targets have been noticed on rare occasions. One slow-moving target was observed during instrument flying weather conditions about August 1, 1952. No interference with traffic has been caused by this problem.
CHICAGO, Midway Airport. Unidentified targets have been seen on many occasions, particularly when temperature inversions have been in effect and low smoke hung over the city. They are usually given as traffic information to other aircraft and occasionally form a nuisance problem, since there is a considerable helicopter activity at and around the airport.
CLEVELAND, Municipal Airport. Unidentified radar targets have been observed many times. The chief controller reported that on a recent occasion such targets moving slowly from west to east showed up in all portions of the scope face.
MINNEAPOLIS, International Airport. No targets of this nature have been reported.
NEW YORK, New York International Airport. No targets of this nature have been reported.
LaGuardia Airport. Only one such instance was reported. At the time it was thought to be due to difficulties within the radar itself.
WASHINGTON, National Airport. Targets of this nature have been observed occasionally over a long period. Recent occasions are logged in Table I of this report.
The history of radar abounds with reports of strange echoes received from supposedly clear skies. Early observers suspected birds or stray weather balloons, but these were eliminated by visual checks. Conjecture that clouds of insects were responsible was also eliminated when such echoes were obtained in the dead of winter. Some connection with the weather was suspected after it was noted that echoes of this type became more numerous on summer nights under calm conditions. Additional evidence indicated that many of these echoes originated in the fine structures of the dielectric (refracting) layers of air-mass boundaries and in regions of air turbulence. Some of the sharpest echoes involved surfaces of pronounced transitions of the water-vapor content of the air. The bibliography at the end of this report contains numerous detailed references to these general phenomena.
August 13-14, 1952.
The observation period started at 1830 Eastern standard time (EST) on the evening of August 13. At the beginning of this period, the moving target indicator was gated to cancel out ground returns up to a range of 10 nautical miles. Beyond this range the scope was clear except for a few permanent echoes that were visible.
Suddenly, at approximately 1957 EST, a group of seven strong stationary targets became visible in an area about 15 miles north-northeast of the radar antenna. During the next two or three antenna revolutions, the area on the scope between Washington and Baltimore became heavily sprinkled with stationary targets in a belt about 6 miles wide. A group of additional targets became visible in an area approximately 10 to 15 miles south of the radar antenna. This was evidence of the beginning of a temperature inversion.
Within the next minute, at approximately 1958 EST. four unidentified moving targets showed up 5 miles southeast of the radar antenna and moved in a southerly direction away from it. When the radar beam was switched from high to low, the targets disappeared. The beam was switched back to high, and the targets returned.
Targets were uniformly small and usually had a weak, fuzzy appearance. However, the target intensity varied from sweep to sweep. Occasionally one or two very strong returns would be received in succession, followed by almost total blanking.
For the next four and one-half hours, many unidentified targets were carefully plotted with a grease pencil on the face of the Type VG scope. The time for each was entered on these plots in order to calculate ground speeds. To secure a permanent record, time data and track plots were transferred from the scope face to a sheet of frosted acetate. These plots are reproduced in Figs. 1, 2, and Fig. 3. The distribution of target ranges is shown in Fig. 4. The average distance that any target was tracked continuously was approximately 2.1 nautical miles.
The observation period was discontinued at 0030 EST on August 14, and steps were taken to secure all available meteorological data relevant to the observation period. The local radiosonde observation which was taken near the midpoint of the observation period, at 2200 EST on August 13, is reproduced in Fig. 5. Winds aloft, as observed at the same time, are listed in Table II.
August 15-16, 1952.
On the night of August 15-16, additional track plots were obtained by Washington ARTC Center personnel. During this period, the radar was operating on the high beam with the moving target indicator gated to 12 miles. The same stationary targets in the Washington-Baltimore belt and in an area 10 to 15 miles south of the radar antenna were visible again on the scope face.
Track plots for this period are shown in Figs. 6 and 7. The local radiosonde observation taken at 2200 EST on August 15 is reproduced in Fig. 8. Winds aloft, as observed at the same time, are listed in Table III.
ANALYSIS OF WASHINGTON DATA
It will be noted from Table I that many more unidentified targets are picked up by the Washington ARTC Center than by the Washington Airport Traffic Control Tower. This may be explained by the fact that the center is equipped with a MEW radar, while the tower is equipped with an airport surveillance radar, Type ASR-1. The most significant differences between the two types of equipment are listed in the following:
1. The peak power of the MEW is 3 decibels (db) higher than the ASR-1. 2. The average power of the MEW is 6 db higher than the average power of the ASR-1. 3. The MEW has a higher elevation angle coverage. 4. The MEW elicits approximately twice as many hits per scan per target since the scan rate of the MEW is 6 revolutions per minute (rpm). Additional specifications of these radars are listed in Table IV.
The almost simultaneous appearance of the first moving targets with the ground returns, signifying the beginning of the temperature inversion, suggested that the target display was perhaps caused by some effects existing in or near the inversion layers.
It will be noted in Figs. 1, 2, and Fig. 3 that all targets observed in the first period were moving from the north or northwest. In Fig. 6 all targets were moving from the south or southwest, and in Fig. 7 all were moving from the west or northwest. The definite directional trend in each case eliminated the possibility that the unidentified targets were
WINDS ALOFT WASHINGTON (SILVER HILL) 22OO EST August 13, 1952
Altitude Direction Velocity
(MSL) (Degrees) (Knots)
Surface Calm 0 1000 Calm 0 2000 350 12 3000 340 12 4000 320 14 5000 320 16 6000 300 18 7000 300 20 8000 310 20 9000 310 22 10000 300 26 11000 290 28 12000 290 29 13000 300 30 14000 300 28 15000 290 29 16000 300 29 17000 300 29 18000 300 30 19000 300 32 20000 300 38 21000 290 38 22000 280 43 23000 280 48 24000 280 50 25000 270 52 26000 280 57 27000 270 61 28000 270 54 29000 270 55 30000 200 62 31000 270 63 32000 280 73 33000 280 84
surface vehicles such as trains, trucks, automobiles, or boats. Had this been the case, some vehicles would have been moving in the reverse directions. In each case, target directions corresponded with the wind
WINDS ALOFT WASHINGTON (SILVER HILL) 2200 EST August 15, 1952
Altitude Direction Velocity
(MSL) (Degrees) (Knots)
Surface 170 5 1000 180 24 2000 190 26 3000 210 24 4000 210 23 5000 220 20 6000 220 16 7000 220 18 8000 220 17 9000 220 13 10000 240 12 11000 270 11 12000 270 13 13000 260 17 14000 260 21 15000 260 25 16000 270 25 17000 270 23 18000 270 22 19000 270 21 20000 260 20 21000 270 22 22000 280 24 23000 290 26 24000 280 26 25000 290 26 26000 300 30 27000 300 34 28000 300 38 29000 290 38 30000 290 36 31000 300 35 32000 300 35 33000 310 34 34000 310 40 35000 300 47 36000 300 49 37000 300 so 38000 300 48 39000 310 42 40000 320 38 41000 300 43 42000 300 53 43000 300 67 44000 310 69 45000 310 60
directions reported aloft. This fact suggested that whatever was producing the targets as being carried by the wind,
The next step of the analysis was to determine, if possible, the altitude of the objects which produced the radar targets. Since the radar actually measures slant range which could in some cases be almost directly overhead from the high-beam MEW antenna, the minimum range of each target was used to determine the absolute maximum altitude of the object producing the target.
TABLE IV RADAR EQUIPMENT CHARACTERISTICS
Tower Radar Center Radar
Type ASR-1 MEW Frequency S-band S-band Pulse-repetition frequency 1,000 900 Pulse rate 0.5 microsecond 1 microsecond Vertical coverage 6,000 feet at 6 miles 12,000 feet at 3 miles Scan Rate 28 per minute 6 per minute Display scopes 12DP7 12DP7 and VG2 Power output 200 kilowatts 400 kilowatts
For example, a target which came within five nautical miles of the radar antenna could not be above an altitude of five nautical miles, or 30,400 feet. With the use of the slant-range principle, the absolute maximum altitude of each target was determined and is listed in Table V. When attempting later to determine the probable altitude of each target by studying the winds aloft, it was useful to have these maximum altitude figures to eliminate the necessity for consideration of higher altitude levels.
Since winds aloft can vary considerably during the period of a few hours, it was decided to use in this analysis only data on targets which were under observation during the periods from one hour before to one hour after the observations of the local winds aloft. These targets are listed in Table V.
During the observation period on the night of August 13-14, all targets on a southerly heading had ground speeds of at least 24 knots. The only reported winds with a southerly heading had a velocity of only 12 knots. These were winds at the 2,000- and 3,000-foot levels. Targets on a southeasterly heading had a speed range of 32 to 48 knots. However, the only winds on this heading were from 14 knots at 4,000 feet to 38 knots at 20,000 feet.
During the August 15-16 observations, targets on a north or northeasterly heading had speeds of 35 to 42 knots. The only reported winds moving in this direction ranged between 5 and 26 knots from the surface up to 9,000 feet. Targets on easterly headings had speeds from 22 to 45 knots. The only reported winds moving in this direction had speeds of from 10 to 24 knots between 10,000 and 25,000 feet.
In Fig. 9 and Fig. 10, the directions and velocities of the winds aloft are plotted on a polar projection diagram together with the directions and velocities of the observed targets. Agreement between the directions of the winds and the directions of the targets is apparent.
One of the theoretically possible causes of the unidentified targets was the delayed pulse or second-time-around effect inherent in the radar method of time measurement. With a second-time-around effect, objects beyond the normal sweep range of a radar can be displayed on the scope because of reception of an echo pulse elicited not by the transmitted pulse which triggers the range sweep but by the preceding transmitted pulse. The apparent velocity of the target on the radar is no greater than and normally less than the velocity of the object producing the return. The heading of the radar target would not necessarily be parallel to the heading of the object unless the object was on a course radial to the radar antenna. These effects are illustrated in Fig. 11.
If we assume then that an object producing a second-time-around radar target was being carried by the wind, the apparent velocity of the target would be no greater than the wind velocity. However, the analysis of the targets listed in Table V showed that
MOVEMENT DATA ON TARGETS TRACKED WITHIN ONE HOUR FROM START OF OBSERVATIONS OF WINDS ALOFT
Date Starting Direction Target Reflector Speed Absolute Maximum Probable Altitude Aug. Time (Degrees) Speed (1/2 Target Altitude (Based on (Based on 1952 EST (Knots) Speed) Minimum Slant Range Winds Aloft)
13 2159 005 28 14 63000 2000 2201 360 24 12 75000 2000 2229 310 33 16.5 23000 8000 2240 300 46 23 30000 9000 2242 325 48 24 33000 9000 2259 010 31 15.5 31000 2000 2303 330 42 21 36000 8000 2330 340 39 19.5 23000 5000 2330 305 39 19.5 24000 8000 2331 315 39 19.5 35000 8000 2332 315 36 18 23000 8000 2345 310 38 19 19000 8000 2347 310 42 21 43000 8000 2349 290 39 19.5 35000 7000 2356 300 42 21 37000 7000 2355 350 36 18 83000 2000
15 2213 260 45 22.5 34000 14000 2226 225 35 17.5 24000 900 2230 250 28 14 37000 10500 2238 185 36 18 29000 900 2240 210 42 21 18000 4500 2353 275 23 11.5 29000 10500*
*This target could also have been a direct radar return from an object floating with the wind at 15000 to 17000 feet mean sea level.
They were actually moving at speeds approximately double the wind velocities reported for the directions involved. This fact eliminated the possibility that the targets were being produced by the second-time-around effect.
When the target velocities plotted in Fig. 9 and Fig. 10 were halved, those plotted points clustered very closely around the wind plots. Further investigation of the doubled-speed effect indicated that this effect could be produced if the original radar beam were reflected downward to give a ground return, as shown in Fig. 12. If we assume that some sort of horizontal reflector was present aloft and that the angle of reflection equaled the angle of incidence of the radar beam, any horizontal movement of the reflector would produce a movement twice as great in the image being received on the radar scope. Furthermore.- the apparent motion of the image would be parallel to the motion of the reflector, as illustrated in Fig. 13.
When the observed target velocities were divided by two, the target motions corresponded closely to the reported wind directions and velocities at certain altitude levels. In nearly all of these cases the altitude levels, which are listed as probable altitudes in Table V, were at or adjacent to the temperature inversion levels.
With only one exception, no targets were seen moving at the speed and heading of the reported wind at any altitude. This suggested that the reflecting areas, which were capable of bending the radar beam, were nevertheless not of sufficient density to produce direct returns on the radar scope. Thus, it appeared likely that the reflection effect was being produced by the atmosphere itself. If this were the case, it would probably be a refraction rather than a reflection which was involved. This effect is shown in Fig. 14.
The uniformly small size of the observed targets as well as the relatively low frequency of their occurrences suggested that the conditions producing this effect were extremely localized and decidedly critical. Although the exact nature of the discontinuity is not known, one possible explanation might be that it is an eddy in the atmosphere. Such eddies may be produced by the shearing effect of dissimilar air masses moving at different speeds and headings at or near the inversion boundary. They might under certain conditions produce bulges in the inversion layer, concentrating and directing the radar energy with a lens effect to produce a return signal strong enough to show up on the radar scope. The relatively short paths of some of the radar targets before their fadeout might be attributed to the dissipation of these eddies in the stratified air mass.
Intermediate speed checks on numerous targets indicated that individual velocities remained quite steady during the observation period. It became possible to predict with accuracy the progress of specific targets from minute to minute. There was no evidence of hovering or of sudden increases in speed by any target. It is believed that previous reports of sudden accelerations of targets to supersonic velocities were due to a controller's transfer of identity from a faded target to another target which was just appearing on a different section of the scope.
It would be unwise to assume that all unidentified slow-moving radar targets are caused by refraction of radar energy. Small rain clouds produce much the same appearance on the scope. Other targets could be direct returns from bird formations, balloons, or debris carried aloft by convection or tornadoes. It has recently been reported that more than 4,000 balloons are released in the United States every day by Government and civilian research organizations. A recent analysis of more than 1,000 visual reports of unidentified flying objects by the Air Technical Intelligence Center at Wright-Patterson Air Force Base indicates that 21.3 per cent of these may be attributed to balloons. 
Examination of the logs of the Washington ARTC Center indicates that there is considerable correlation between the appearance of unidentified targets on the radar scope and the receipt of numerous visual reports of flying saucers. It should be noted that abrupt temperature inversions aloft can refract light in much the same way as radar waves and produce mirage effects. In a standard reference work on meteorology.  Humphreys reports that a temperature inversion (near the surface) of 1 deg. C per meter bends down a light ray into an arc whose radius is 0.16 that of the earth; an inversion of 10 deg. C per meter gives an arc radius of 0.016 that of the earth, or approximately 60 miles. This effect makes it possible for an observer to see in the sky the sun or some other bright light that is actually well below the observer's horizon. On rare occasions, multiple images of the same object may be visible. It is believed that many visual sightings of flying saucers can be explained by this phenomenon.
SUPPLEMENTARY OBSERVATIONS AT INDIANAPOLIS
November 4, 1952.
During test runs of the new ASR-2 radar equipment, a large number of unidentified moving targets appeared on the scope at approximately 4 p.m. The sun was low in the sky, and the sky was clear of all clouds. Ceiling and visibility were unrestricted. Pilot temperature reports from a departing aircraft indicated that a pronounced temperature inversion existed at the 6,000-foot level.
Although no targets were plotted, a check on several indicated that their movement corresponded to the direction of the wind at the inversion level, with a velocity roughly double the wind velocity. Targets were larger, stronger, and more numerous than those observed by the writers during the Washington observations. At times the clutter made it difficult to keep track of actual aircraft targets on the scope.
November 5, 1952.
At approximately 4 p.m., a group of similar targets appeared on the Indianapolis ASR-2 scope. Again the sky was clear of clouds; ceiling and visibility were unrestricted. Targets were strong. numerous, and of various shapes and sizes.
A simultaneous check of the L-band radar showed that only a few targets were being picked up by this equipment. The L-band targets appeared considerably weaker than those seen on the ASR-2 scope, although L-band aircraft targets appeared normal.
By manipulation of the ASR-2 antenna motor switch, it was possible to slew the antenna to beam it directly at some of the unidentified targets. The video return was displayed on an A-scope for closer analysis of the target characteristics. Comparisons were made with the A-scope characteristics of aircraft targets.
Aircraft targets showed sharp rise a "decay times as well as relatively constant shape and amplitude. The unidentified targets showed gradual rise and decay times, amplitude and shape showed wide variation which resulted in a random interlaced sign envelope similar to that returned by rain and cloud formations. These target characteristics are sketched in Fig. 15.
ANALYSIS OF SUPPLEMENTARY OBSERVATIONS
The reduced target returns from the L-band radar indicated that the reflecting areas are formed by atmospheric disturbances or discontinuities rather than by some form of ionization. If the cause were ionization, it would be expected that the lower frequency of the L-band equipment would increase the susceptibility of the radar energy to reflection or refraction effects. An example of this trend is that of ionospheric layers which produce no appreciable reflection of ultra-high-frequency energy but cause strong skip propagation of the lower radio frequencies.
EFFECT ON AIR TRAFFIC CONTROL OPERATIONS
The generally weak and fuzzy appearance as well as the slow speed of spurious radar targets usually enable them to be recognized as such by experienced radar controllers. Normally these targets have but little effect on traffic control, because they occupy very little space in relation to the entire scope area and their progress on course is very slow. The most dangerous possibility from the traffic control standpoint is the chance that one of these targets might be a helicopter.
If their course will not collide with that of an aircraft target, such targets are generally disregarded. If the course will collide with an aircraft target, some control action is indicated because of the helicopter hazard. In such cases, prudent controllers will give traffic information to pilots regarding the unidentified target, particularly at night under visual flight rule conditions. Where a collision course is involved, pilots would rather be warned about a spurious target than not be warned about a legitimate one.
At the present time, very little instrument flying is done by helicopters. Therefore, unidentified targets of this type are not usually given as traffic information to pilots known to be operating on instruments.
1. It is believed that most of the unidentified targets observed on the Washington MEW radar during the period beginning on the night of August 13, 1952 and the period beginning on the night of August 15, 1952 were ground returns caused by reflection phenomena closely connected with the temperature inversions in the lower atmosphere. 2. Unidentified radar targets of the type described in this report have been noticed since the early days of radar. Unusual weather conditions prevailing in the Washington area during the summer of 1952 were exceptionally conducive to the formation of these phenomena. 3. Present evidence indicates that the appearance of unidentified targets of this nature on radar scopes has but little effect on the control of air traffic. At its worst, it forms a nuisance by cluttering the scope display and by requiring that additional traffic information or heading instructions be issued in order to protect other traffic against the possibility that such a target might be a helicopter. 4. In some cases, it would be desirable to provide the controller with a more positive method of identifying targets such as these so that he could determine quickly whether they are spurious or whether they are actual aircraft.
1. In order to secure additional evidence regarding the causes, extent, and effects of this type of phenomena, it would be desirable to secure additional target plots from the horizontal plotting scope of the Washington ARTC Center. It would also be desirable for all CAA air traffic control agencies which use radar equipment to log the occurrence of such targets. Notes regarding the extent and motion characteristics of them, together with their effects on the control of air traffic, would also be of value. It would be desirable to correlate all these reports with official United States Weather Bureau records. 2. Should additional research regarding these phenomena be undertaken, close coordination with the local office of the United States Weather Bureau is essential in order that observations can be made when conducive meteorological conditions are expected. 3. It is believed that more complete evidence could be obtained through the use of more flexible radar equipment. A tremendous asset in evaluating the nature of false targets would be the ability to track continuously a specific target through use of a manual slewing control. It would then be desirable to examine this target closely on an A-scope radar presentation. A number of commercially available synchroscopes are ideally suited for this purpose. The echo could be enlarged on such a presentation to a width of one inch or more. Examination of the resulting trace including such characteristics as steepness of rise and decay time, energy distribution, and fluctuations in amplitude should make it possible to deduce a great deal regarding the source of the reflection. 4. Additional simultaneous observations of the phenomena on L-band and S-band radar equipment would be desirable, The availability of aircraft which could be guided by radar to the area of the target or to the primary reflecting area would also be advantageous. Additional information maybe obtained by equipping the aircraft with an aero-psychograph as well as with suitable apparatus for measuring electrical charges in these areas. 5. When helicopter traffic becomes more prevalent, it may be desirable to provide the controller with some type of radar accessory which can detect propeller modulation and which can give him the means to determine positively whether an unidentified target is an aircraft or a reflection. It is recommended that this type of accessory be studied in connection with the proposed evaluation program for the ASR-2 radar.
"Echoes from the Atmosphere," Bell Laboratories Record, Vol. 25, No. 2, Feb. 1947, p. 5.
Friend, A.W. "Theory and Practice of Tropospheric Sounding by Radar," Proceedings of the Institute of Radio Engineers, Vol. 37, No. 2, Feb. 1949. p. 116.
Friis, H. T,. "Radar Reflections from the Lower Atmosphere," Proceedings of the Institute of Radio Engineers, Vol. 35, No. 5, May 1947, p. 494.
Goldstein, Herbert, "Origin of the Echo," in "Propagation of Short Radio Waves," Massachusetts Institute of Technology Radiation Laboratory Series, McGraw-Hill Publishing Company, New York, 1951, Vol. 13 edited by Donald E. Kerr, Chapter 7, pp. 593-595.
Gordon, W. E., "A Theory of Radar Reflections from the Lower Atmosphere," Proceedings of the Institute of Radio Engineers, Vol. 37, No. 1, Jan. 1949, p. 41.
Gould, William B., "Radar Reflections from the Lower Atmosphere," Proceedings of the Institute of Radio Engineers, Vol. 35, No. 10, Oct. 1947, p. 1105.
Humphreys, W. J., "Physics of the Air," McGraw-Hill Publishing Cornpany, New York, 1940.
"Many Potential 'Saucers,' " Science News Letter, Vol. 62, No. 7, Aug. 16, 1952, p. 106.
"Radar Returns from the Lower Atmosphere Viewed on an AN/GNP-2 Screen," Landing Aids Experiment Station Progress Report, 1948 Test Section, Electronics, pp. 22-24.
 Many Potential "Saucers,", Science News Letter, Vol. 62, No. 7, Aug. 16, 1952, p. 106.  "Unidentifted Aerial Objects Receive Careful Analysis by Air Force Experts," The Aircraft Flash, published by Department of the Air Force, Air Defense Command, Vol. 1, No. 4, Jan. 1953, p. 4.  Humphreys, W. J., "Physics of the Air," McGraw-Hill Publishing Company, New York, 1940.
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